An internal combustion engine such as a gasoline engine and a diesel engine are used as a prime mover of a vehicle. The internal combustion engine starts running autonomously when ignited while being rotated at a predetermined speed and delivering the fuel thereto. That is, it is unnecessary to deliver the fuel to the engine in case the engine is being rotated at a speed higher than a reactivatable lower limit speed at which the engine is allowed to rotate autonomously by delivering the fuel thereto, unless driving force is demanded. Therefore, the conventional control systems are configured to carry out the fuel-cut control utilizing the above-explained property for the purpose of improving fuel economy. Specifically, the fuel-cut control is carried out by stopping the delivery of fuel to the engine under the condition in which the vehicle is being decelerated, and the engine is thereby rotated compulsory by an inertia force at the speed higher than the reactivatable lower limit speed.
Thus, the fuel-cut control is carried out under the situation in which the drive force is not demanded, that is, the accelerator is not depressed, and the engine is rotated at the speed higher than the reactivatable lower limit speed. Therefore, in order to improve fuel economy, it is preferable to carry out the fuel-cut control as long as possible by keeping the speed of the engine higher than the reactivatable lower limit speed as long as possible during deceleration of the vehicle. For this purpose, in case the vehicle comprising a torque converter having a lockup clutch, the engine speed is prevented from being lowered during deceleration by keeping the lockup clutch to be engaged within an acceptable range of noises and vibrations. In this situation, since the engine speed is lowered by decelerating the vehicle, a speed change ratio of the vehicle is increased gradually to a low speed side.
An example of the control system for a continuously variable transmission having a lockup clutch, which is configured to carry out the above-explained control during execution of the fuel-cut control is disclosed by Japanese Patent Laid-Open No. 01-303356. Specifically, the controller taught by Japanese Patent Laid-Open No. 01-303356 is applied to a continuously variable transmission comprising a lockup clutch connecting an input side member and an output side member of a fluid coupling. The controller taught by Japanese Patent Laid-Open No. 01-303356 is configured to lower a target torque ratio of the continuously variable transmission in case a slippage of a drive wheel is detected thereby eliminating the slippage, and according to the teachings of Japanese Patent Laid-Open No. 01-303356, the lockup clutch is disengaged when executing the above-explained control. However, if the lockup clutch is disengaged, the torque is transmitted through the fluid coupling. As a result, a transmission capacity of the torque between the input member and the output member is changed, and a driving torque is thereby varied. Therefore, shocks may be caused in the continuously variable transmission.
Meanwhile, a control device for controlling the lockup clutch during deceleration of a vehicle is disclosed by Japanese Patent Laid-Open No. 2002-234340. Specifically, the control device taught by Japanese Patent Laid-Open No. 2002-234340 is configured to extend an execution time of the fuel-cut control by engaging the lockup clutch to keep the engine speed to a relatively high speed during deceleration of the vehicle. That is, the control device taught by Japanese Patent Laid-Open No. 2002-234340 is configured to keep the engine speed higher than the reactivatable lower limit speed. For this purpose, according to the teachings of Japanese Patent Laid-Open No. 2002-234340, a load of an air conditioner is reduced to prevent a drop in the engine speed, during execution of the fuel-cut control while engaging the lockup clutch. In addition, according to the teachings of Japanese Patent Laid-Open No. 2002-234340, a lockup cancellation speed is set to prevent an uncomfortable feeling resulting from deceleration increased by disengaging the lockup clutch.
In addition, another control device for controlling the lockup clutch during deceleration of a vehicle is disclosed by Japanese Patent Laid-Open No. 2006-342899. Specifically, the control device taught by Japanese Patent Laid-Open No. 2006-342899 is configured to prevent an abrupt rise in engine speed when restarting the vehicle, and to prevent an engine stall when braking the vehicle abruptly. For this purpose, according to the teachings of Japanese Patent Laid-Open No. 2006-342899, a lockup pressure for engaging the lockup clutch is raised in case a required time until the vehicle is stopped is long, e.g., in case the vehicle speed is relatively high. To the contrary, the lockup pressure is lowered in case a required time until the vehicle is stopped is short, e.g., in case the vehicle speed is relatively low.
As described, the fuel-cut control is carried out under the situation in which the engine is rotated compulsory by the inertia force of the running vehicle. In this situation, therefore, a friction loss and a pumping loss of the engine act as a braking force. Therefore, a control device disclosed by Japanese Patent Laid-Open No. 2006-143000 is configured to utilize the friction loss and the pumping loss as the braking force under the situation in which the vehicle is coasting. According to the teachings of Japanese Patent Laid-Open No. 2006-143000, the friction loss and the pumping loss used as the braking force are reduced in case a downshifting is carried out under the situation in which the vehicle is coasting.
In addition, Japanese Patent Laid-Open No. 2000-272381 discloses a control system, which is configured to cooperatively carry out a control of a speed change ratio to keep the engine speed to a relatively high speed, and a control of the pumping loss of the engine. According to the teachings of Japanese Patent Laid-Open No. 2000-272381, in case the fuel-cut control is carried out during deceleration of the vehicle, the speed change ratio is increased gradually to keep the engine speed higher than the reactivatable lower limit speed. In this situation, the pumping loss of the engine is reduced to prevent an increase in deceleration resulting from thus increasing the speed change ratio, by increasing an opening degree of an accelerator gradually, while reducing a load resulting from operating an air conditioner.
In case of carrying out the fuel-cut control, an output torque of the engine is lowered. To the contrary, in case of restarting the delivery of fuel to the engine, the output torque of the engine is increased. Therefore, Japanese Patent Laid-Open No. 2001-182584 discloses a control device for preventing an occurrence of shocks resulting from such fluctuation of the engine torque. Specifically, the control device taught by Japanese Patent Laid-Open No. 2001-182584 is configured to reduce torque down shock resulting from execution of the fuel-cut by reducing an amount of torque reduction resulting from execution of the fuel-cut. For this purpose, according to the teachings of Japanese Patent Laid-Open No. 2001-182584, a generating amount of an alternator is reduced and an opening degree of an accelerator is increased before starting the fuel-cut control. The control device taught by Japanese Patent Laid-Open No. 2001-182584 is also configured to reduce torque up shock resulting from terminating the fuel-cut by increasing generating amount of the alternator while reducing an opening degree of the throttle valve.
As described, in order to prevent an uncomfortable feeling resulting from the deceleration, the control device taught by Japanese Patent Laid-Open No. 2002-234340 is configured to set the lockup cancellation speed. Therefore, according to the teachings of Japanese Patent Laid-Open No. 2002-234340, the lockup clutch has to be disengaged at a relatively high speed. For this reason, the execution time of the fuel-cut control may be shortened and a fuel saving effect may be degraded. In addition, when the lockup clutch is disengaged, negative torque acting on the drive wheel is reduced thereby causing shocks. Further, the delivery of fuel to the engine is restarted and the engine is thereby driven to output the torque when the lockup clutch is disengaged. In this situation, therefore, the shocks may be amplified by such activation of the engine and the disengagement of the lockup clutch.
As also described, the control device taught by Japanese Patent Laid-Open No. 2006-342899 is capable of preventing an abrupt rise in engine speed and an engine stall resulting from a delay of disengagement of the lockup clutch. However, although the lockup clutch is disengaged when the vehicle is decelerated, the driving torque is fluctuated before and after the disengagement of the lockup clutch. Therefore, the shocks may be amplified by such disengagement of the lockup clutch.
In addition, the control devices taught by Japanese Patent Laid-Opens No. 2006-143000, No. 2000-272381, and No. 2001-182584 are also capable of reducing a torque fluctuation resulting from execution of the fuel-cut and the downshifting for keeping the engine speed. As described, according to the teachings of those prior art documents, the engine speed is kept to a relatively high speed by engaging the lockup clutch while the vehicle is coasting, that is, during deceleration of the vehicle. However, if the lockup clutch is disengaged when the vehicle is further decelerated in this situation, a transmission torque capacity of the torque converter is varied. As a result, shocks may be amplified by such variation in the transmission torque capacity of the torque converter.